The present invention relates generally to molds for producing textured articles and more particularly to insulated molds with a selectively etchable multilayered top layer for uniformly texturing molded articles.
Molding of thermoplastic resins is a promising technique for producing relatively thin, wide and strong plastic parts such as panels for use in automobiles or appliances. Depending on specific requirements, such plastic parts can be made by any of a number of molding processes such as blow molding, injection molding with cold or hot runners and with gas assist, and compression molding.
Blow molding involves the extrusion of a molten tube of resin called a parison into a mold. The mold closes around the parison, pinching the bottom of the parison closed. A gas such as air is then introduced causing the tube to expand against the cool surfaces of the mold. When the parison comes into contact with the cool mold surface, the plastic at the surface quickly freezes. This results in surface imperfections such as die lines, fold lines, pores and voids.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal material such as iron, steel, stainless steel, aluminum alloy or brass. Such materials are advantageous in that they have high thermal conductivity and thus allow the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. However, because of the rapid cooling, the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates several problems, particularly when molding resins which contain large amounts of fillers in the form of fibers and powders. The freezing of these materials at the mold surfaces creates rough surfaces such as exposed fillers, voids and porosity. Processing difficulties arise when producing thin, large parts. The quick solidification of the melt combined with limited flowability of the materials makes it difficult to achieve melt flow over a large area. The use of multiple gates for large and/or complex mold cavities produces weld lines, which are unsightly and weak. Another important issue in injection molding of high quality parts is the residual stresses in the molded parts. Residual stress inside a part can result in dimensional instability over the lifetime of the part. Non-uniform residual stresses also produce differential refractive indices. The dimensional stability and uniformity of refractive indices are critically required for high quality parts.
Compression molding processes of glass reinforced thermoplastic sheets begin with heating the composite blanks. The material is heated above its melting point or if an amorphous material at least substantially above its glass transition temperature. When the composite blanks are heated, they expand (loft) due to the recoil forces within the fibers. The hot blanks are then pressed between cool mold surfaces which are below the melting point or glass transition temperature (typically 175.degree.-250.degree. F.). Contact with the cool mold surfaces results in frozen resin on the surface of the blank. This creates unfilled areas in the form of exposed fibers and surface porosity. Since the resin at the cold surface is frozen and does not flow, rough boundaries between the charged and newly formed areas are produced.
In injection compression molding which is a combined process, a hot thermoplastic melt is injected into a mold cavity. The parting line of the mold is positioned open or allowed to be forced open by the injected melt typically 0.05" to 0.3" inches. The clamping force is increased initiating the compression stroke of the mold forcing the melt to fill the cavity. In many instances the velocity of the melt front through the cavity changes as the injection stroke stops and the compression stroke begins. This distinct change in melt front velocity is often characterized by a stall followed by a surge in the melt front.
The melt begins to quench on the cavity walls as it is injected into the mold. As the melt front stalls, at the completion of injection, and then surges forward, upon the initiation of compression, a blemish, sometimes referred to as a halo, may be produced in the surface of the molded article. The blemish is the result of differential cooling and shear stress which occurs in the injection compression process as a result of the melt front velocity change.
There have recently been disclosed multilayer molds in which a metal core has an insulating layer bonded thereto, for the purpose of slowing the initial cooling of the resin during the molding operation. The insulating layer is fabricated of material having low thermal conductivity, thus slowing the cooling of the molten resin, and also having good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. Said layer may be made of a resin such as polyimide, polyamideimide, polyethersulfone or polyetherketone, typically applied in uncured form (e.g., as a polyamic acid in the case of a polyimide or polyamideimide) and subsequently cured. Cured resins in a solvent carrier may also be employed.
One important requisite for the use of molded plastic parts in large, exterior panel applications is a finished surface quality. The surface of molded plastic parts should be as finished as that of current exterior parts made from sheet metal. Improved surface quality of molded plastic parts has been achieved by means of molds in which a polymeric insulating layer is disposed on the mold core and a durable thin skin metal layer is disposed on the insulating layer. Due to the insulation, the skin layer retains heat during the molding operation, thereby avoiding the surface irregularities created by rapid surface cooling. Thus, these devices provide a finished surface while maintaining a relatively short cycle time.
Exemplary insulated molds have an insulating layer of about 0.01 inch and a corrosion-resistant nickel outer skin. The application of the insulating layer involves spraying the polymer in a solvent solution and drying. This is repeated until the final desired coating thickness has been achieved after which a layer of Ni is electro deposited or applied electrolessly.
Particularly advantageous multilayer mold structures have been produced in which the mold is first coated with a thermal insulator, such as polyamideimide, and then overcoated with one or more layers of an abrasion-resistant metal such as electroless nickel. In some instances the outer layer of the polymer has a porous primer layer formed of a matrix of nickel particles suspended in the polymer to promote durability of the mold and to promote adhesion of the nickel skin layer.
In a conventional mold, a texture such as leather grain, is created by selectively etching the pattern into the steel mold cavity. Textured depths typically vary from about 0.0004 inches to as deep as 0.01 inches. During the coating process the insulator flows a small amount, washing out the texture in the steel mold cavity, thereby resulting in an unacceptable texture which is not sharp. Thus, it is necessary to etch the mold surface layer in order to achieve good texture definition.
The multilayer structure presents at least two significant problems. First, it is known in the art that the durable skin layer usually formed of high phosphorous nickel (P&gt;8%) is difficult to etch, and results in non-uniform removal of material. Thus, it is not done commercially. Also, the resists used in the industry will not survive the strong chemical etchants required. Secondly, the etchant used in texturing may not be allowed to penetrate to the insulating layer, especially a nickel filled layer. If penetration occurs, the etchant attacks the nickel particles and the polymer, resulting in delamination between the nickel skin and the insulator. To use such a technique would then therefore risk a catastrophic failure of the mold, requiring reprocessing, which is expensive and time-consuming. There thus exists no acceptable method for texturing the molds to produce acceptable textured articles. Accordingly, a high quality, reproducible textured mold having an insulating layer, and a durable mold surface producing finished molded articles is desired.